Until relatively recently, standard coin operated telephones contained carbon microphones in their handsets. These carbon microphones passively balanced the line and pulled 5 milliamperes (mA) or more during their normal operation.
With the advent of deregulation of the telephone industry, a demand arose for low power telephone controllers incorporating various features, and a number of phones were introduced which employed dynamic microphones. Although dynamic microphones are more expensive than carbon microphones, they offer better fidelity. Further, as long recognized in the prior art, the usage of a dynamic microphone required the addition of circuitry to amplify the microphone output and dynamically balance the line. In one highly advanced low power electronic controller, the LES 100 WE Electronic Payphone Retrofit Kit, developed and sold by Mars Electronics, the overall power constraints on the design of the controller did not permit the steady usage of a current of 5 mA or higher as typically employed by a carbon microphone. Consequently, the carbon microphone was replaced with a dynamic microphone. While this microphone did not require any external current supply to operate, a current of approximately 100 microamperes (uA) was used to detect the presence or absence of this microphone. The Mars Electronics LES 100 controller is substantially as described in U.S. application Ser. No. 07/199,129 filed May 26, 1988, now U.S. Pat. No. 4,926,458, and assigned to the assignee of the present invention.
While that controller has enjoyed immediate commercial success as a retrofit for use with standard Western Electric payphones, its installation has required a modification of each standard payphone being retrofit to replace its standard carbon microphone with a more expensive dynamic microphone.